The present invention relates generally to a vehicle anti-skid braking system, and more particularly to a hydraulic pressure control device having means for giving the driver an appropriate brake pedal reaction in such a vehicle anti-skid braking system.
Various anti-skid braking systems for use in motor vehicles have been proposed and used for optimizing a hydraulic braking pressure under the control of a control unit to provide a more effective and safer braking action when the vehicle is braked in different road conditions. U.S. Pat. Nos. 4,090,739 and 4,278,300, for example, illustrate such anti-skid braking systems. However, prior anti-skid braking systems have proven unsatisfactory in that the brake pedal is subjected to a kickback during operation of the system. More specifically, when the directional control valve is shifted under the control of the control unit to reduce the braking fluid pressure in a pressure-decreasing mode, the braking fluid is discharged from the brake actuating cylinders in vehicle wheels into a reservoir, from which the braking fluid is pumped by a hydraulic pressure pump back to a supply pipe connected to the master cylinder. When the wheels are in the anti-skid control mode while they are being braked, the braking fluid discharged from the brake actuating cylinders and pumped back to the master cylinder acts on the brake pedal as a kickback, giving the driver a bad pedal feeling.
To solve the above problem, various efforts have been made to prevent the braking fluid discharged into the supply pipe from flowing back into the master cylinder when the directional control valve is in the pressure-decreasing mode. One prior arrangement for achieving such a function is disclosed in Japanese Laid-Open Patent Publication No. 56-142733 published on Nov. 7, 1981.
The disclosed hydraulic pressure control device has proven effective in completely eliminating the undesirable kickback and also any pedal reaction to be felt by the driver's foot. One problem with this prior hydraulic pressure control device is that since there is no pedal reaction whatsoever during the anti-skid control mode, the driver cannot confirm whether the vehicle wheels are under the anti-skid control or not.